Billy - You misread or I wasn't clear enough...what occupies that portion
of space??
Atmosphere?? Earth?? or the 'apparent' nothingness of space where matter,
either air or other is simply not detectable.
So, I meant what is present in that particular space, in this case the
space of Mars.
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Dave, as to friction, think about it and its 'accepted' definition.
Remember where you are and open up to different approaches....this requires
lateral thinking.